Liquefied gases, such as liquid nitrogen, liquid oxygen, helium, hydrogen and etc., are stored at cryogenic temperatures within mobile storage tanks for shipment to customers. Such mobile storage tanks are incorporated into truck drawn trailers, rail cars, and sea going tanks.
Although such mobile storage tanks are heavily insulated and contain layers of vacuum insulation, heat leakage by radiant heat transfer causes vaporization of the liquefied gas during shipment. The warm vaporized gas is vented and thereby results in product loss. In order to minimize product loss through vaporization, certain mobile storage tanks used for the transport of value added products such as helium can have thermal shields that are designed to minimize product loss through such vaporization. The thermal shield is a conductive metal sheet that overlies the top of the product container and is located in a surrounding vacuum insulation layer. A shield fluid tank containing a cryogenic shield fluid, such as nitrogen in case of helium, is connected by piping to the thermal shield. Radiant heat energy that would otherwise pass to the product container holding the product is intercepted to inhibit the radiant heat transfer to the container. There are three types of shields: a single shield with a single shield fluid or gas; a single shield with two shield fluids or gases; two shields, each shield having a different single fluid or gas.
During long journeys, the shield fluid or gas is vented through the thermal shield to allow liquid thermal shield fluid or gas to enter piping of the thermal shield. The liquid or cold gas that replaces the warm gas will eventually be vaporized and further replaced. As the time of transit increases, the product is also vented. In this regard, in mobile storage tanks designed for helium transport, a helium thermal shield is provided that underlies the nitrogen thermal shield.
The current practice to control product loss by venting is to control the venting by a pressure regulator or flow meter and sometimes a pipe orifice. Typically, the pressure regulator or flow meter is preset in accordance with a schedule that depends on the predicted length of transit. The settings of the pressure regulators or flow meters are not changed after departure because such setting has to be effected by trained personnel.
As may be appreciated, due to unforeseen delays, mobile storage containers sometimes arrive at their destinations with a high percentage of product loss or empty. For instance, during a lengthy ocean voyage, where the storage tank is in customs for too long a time, all the product may be lost. In case of helium, this can be a particularly expensive loss.
The monitoring of liquefied gas storage within storage tanks at fixed installations is a relatively straightforward proposition. In such storage tanks, liquid level is sensed, recorded and transmitted by a remote telemetry unit, which has either a cell phone wireless communication modem or a standard land line modem. The data is read at a central station for managing the particular storage tank. Low liquid level can be addressed by simply refilling the storage tank with an amount of liquid calculated to meet a customers needs.
The problem with implementing such a system with respect to a mobile storage tank is that by necessity, the mobile storage tank can only be provided with on-board battery and/or solar power. Thus, the use of a remote telemetry unit in the same manner as in a fixed installation is not practical. Additionally, even with the use of a remote telemetry unit, mere transmission of pressure, content level, and/or temperature data are not very useful if the location of the mobile storage tank is not known. For instance, the actions that could be taken by a gas supplier with respect to the mobile storage tank being shipped by oceanic transport would be different depending upon whether the tank were in the port and delayed in customs versus in transit and on the open sea.
As will be discussed, the present invention provides a remote monitoring system for a mobile storage tank that utilizes a remote telemetry unit in such a way that data is only periodically recorded and transmitted so that battery power may be conserved and that in preferred embodiments can exercise a control function to selectively vent heat fluid vapor and/or product vapor.